A laser interferometer having a laser to produce a beam of light and a first beam splitter to split the beam of light into first and second portions. The first portion is directed towards a measuring interferometer and the second portion is directed toward a first regulating interferometer having a regulating section. A beam director for directing a portion of the second portion of the beam of light toward a second regulating interferometer having a regulating section. A switch connected to the second regulating interferometer which turns the second regulating interferometer on when a first output signal is less than a first predetermined maximum signal threshold value or exceeds a first predetermined minimum signal threshold value and if the second regulating interferometer has the next largest regulating section length relative to the length of the regulating section of the first regulating interferometer. The switch also turns the second regulating interferometer on when the first output signal exceeds a second predetermined maximum signal threshold value or is less than a second predetermined minimum signal threshold value and if the second regulating interferometer has the next smallest regulating section length relative to the length of the regulating section of the first regulating interferometer. A control device for controlling the wavelength of the beam of light in response to the output control signal corresponding to the regulating interferometer switched on at that given time.

A laser interferometer apparatus is designed such that the path of the laser beam of an interference refractometer to the path of the laser beam of a measuring interferometer is covered with a blast duct temperature-controlled atmospheric gas blown by a blower fan is directed through the interior of the blast duct toward the corner cube of a moving stage along the laser beam.

An absolute measuring interferometer having a measuring interferometer, a tunable laser emitting a laser beam and a control interferometer for adjusting the air wavelength of the laser beam. The control interferometer adjusts the air wavelength of the laser beam to a specific wavelength value at the ends of each measuring cycle. The wavelength of the tunable laser is continually tuned within the specific wavelength interval where the phase change of the interference signal is continually detected during the wavelength modulation process. An absolute measurement is determined by a simple mathematical relationship between the measured wavelength and phase changes.

An apparatus and method for measuring absolute measurements having two measuring interferometers and a tunable laser emitting a laser beam. The two measuring interometers each have their own measuring lines and are supplied with the beam from one and the same laser. A reference line is established from the arithmetic sum or difference of the two measuring lines and is maintained at a constant value.

In this invention, in order to realize an interferometer apparatus which can correct a measurement error caused by a change in refractive index due to a fluctuation of a gas such as air with high precision, and can assure a high-precision, stable measurement, first and second interferometer means are arranged, and two measurement reflection means are integrally moved while a predetermined relationship is held between measurement optical path lengths and reference optical path lengths formed by the two interferometer means.